As is known in the art, the transmission path used for transmitting signals over data links causes interference to telecommunications. This occurs irrespective of the physical form of the transmission path, i.e. whether the transmission path is a radio connection, an optical fibre or a copper cable. Especially in radio communications, situations occur, in which the quality of the transmission path varies from one connection to another and also during a connection.
A typical phenomenon is fading occurring on the radio path and causing changes to a transmission channel. Other concurrent connections may also cause interference, which may vary as a function of time and place.
In a typical radio communications environment, signals between a transmitter and a receiver propagate over several paths. Such multipath propagation is mainly caused by a signal being reflected from surrounding surfaces. Signals that have propagated along different paths arrive at the receiver at different times owing to different propagation time delays. Different methods have been developed to compensate for the fading caused by the multipath propagation.
A solution to the problem is to use diversity in the transmitter. Time diversity employs interleaving and encoding to achieve time-based diversity in the signal to be transmitted. However, delays in transmission present a drawback, especially when the channel is slowly fading. In frequency diversity, in turn, the signal is transmitted simultaneously at several frequencies. This is nevertheless an inefficient method, when the channel is provided with a wide coherence bandwidth.
Antenna diversity employs more than one antenna for transmitting and/or receiving a signal. Thus, the signal components that have multipath-propagated through different channels will probably not be interfered with by a simultaneous fade. In reception diversity, two or more antennas having a deviating location are used to receive the transmitted signal. A drawback with the reception diversity is that the use of two antennas is difficult to implement in a terminal, which is to be small. In transmit diversity the same signal is transmitted to a receiver using two or more different antennas. The receiver combines the signals for instance by means of MLSE (maximum likelihood sequence estimator) or MMSE (minimum mean square error) methods. Transmit diversity more applicable to the downlink direction of mobile communication systems than reception diversity, as it is easier to provide a base station with several antennas than a terminal.
Diversity methods may also utilize feedback. What is known as “closed-loop” diversity is then concerned, in which the receiver signals information to the transmitter and in which the general aim is to maximize the signal-to-noise ratio in the receiver. If no feedback is used, then the diversity concerned is referred to as “open loop” diversity.
Cellular radio systems currently under development, such as UMTS, provide the possibility to use two transmit antennas. An example thereof is the UMTS standard version “Release 99”. However, the use of even more antennas, for instance four antennas, in transmit diversity is also being developed. Such methods can possibly be employed in subsequent UMTS standard versions. According to Release 99, when transmit diversity is applied, each channel is transmitted with two radiation patterns.
The current UMTS standard employs Space-Time Transmit Diversity (STTD) with encoding ratio 1 applicable to two transmit antennas. The symbols to be transmitted are grouped into blocks, S1 and S2, comprising two symbols. The encoding is determined in its basic mode by a 2×2 matrix:
                              C          ⁡                      (                                          S                ⁢                                                                  ⁢                1                            ,                              S                ⁢                                                                  ⁢                2                                      )                          =                  [                                                                      S                  ⁢                                                                          ⁢                  1                                                                              S                  ⁢                                                                          ⁢                  2                                                                                                                          -                    S                                    ⁢                                                                          ⁢                                      2                    *                                                                                                S                  ⁢                                                                          ⁢                                      1                    *                                                                                ]                                    (        1        )            where * denotes a complex conjugate. This matrix extends the encoding over two symbol periods.
If more than two transmit antennas or radiation patterns are to be used, other encoding solutions have to be developed. Publication V. Tahork, H. Jafarkhani, A. R. Calderbank “Space-time block encoding for wireless communications: theory of generalized orthogonal designs” IEEE Trans. Inf. Th., 1999 discloses a solution offering full diversity. The encoding ratio of the presented code is, however, only ¾. In addition, as to the efficiency, the solution is not in balance: the power transmitted from different antennas varies in the different symbol time slots.
Publication O. Tirkkonen, A. Boariu, A. Hottinen: “Minimal orthogonality space-time block code for 3+ Tx antennas”, Proc. IEEE Int. Symp. Spr. Spectr. Tech. Appl. (ISSSTA), New Jersey, USA, September 2000 discloses a solution applicable to three or four antennas or radiation pattern further developed based on the matrix (1). The encoding is determined by the following matrix:
                              C          ⁡                      (                                          S                ⁢                                                                  ⁢                1                            ,                              S                ⁢                                                                  ⁢                2                            ,                              S                ⁢                                                                  ⁢                3                            ,                              S                ⁢                                                                  ⁢                4                                      )                          =                  [                                                                      C                  ⁡                                      (                                                                  S                        ⁢                                                                                                  ⁢                        1                                            ,                                              S                        ⁢                                                                                                  ⁢                        2                                                              )                                                                                                C                  ⁡                                      (                                                                  S                        ⁢                                                                                                  ⁢                        3                                            ,                                              S                        ⁢                                                                                                  ⁢                        4                                                              )                                                                                                                        C                  ⁡                                      (                                                                  S                        ⁢                                                                                                  ⁢                        3                                            ,                                              S                        ⁢                                                                                                  ⁢                        4                                                              )                                                                                                C                  ⁡                                      (                                                                  S                        ⁢                                                                                                  ⁢                        1                                            ,                                              S                        ⁢                                                                                                  ⁢                        2                                                              )                                                                                ]                                    (        2        )            
The prior art solutions are, however, not able to provide a fully satisfactory final result when more than two radiation patterns are used or if the transmitter is provided with partial information about the transmission path parameters.